The present invention relates to an electron gun for a color cathode-ray tube and, more particularly, to an electron gun for an inline color cathode-ray tube capable of displaying an image at a high resolution.
A BPF (BiPotential Focus) electron gun like the one shown in FIG. 1 has conventionally been known as a general electron gun for a color cathode-ray tube.
As shown in FIG. 1, this BPF electron gun is composed of three cathodes KR, KG, and KB including heaters and aligned in line, and a plurality of electrodes, i.e., first, second, third, and fourth grids G1, G2, G3, and G4 sequentially arranged from the cathodes direction in which these cathodes emit electron beams.
In the BPF electron gun having this arrangement, an electron beam is generated by a triode portion made up of the cathodes KR, KG, and KB, the first grid G1, and the second grid G2. The electron beam is finally focused on a phosphor screen by a prefocus lens portion formed by the second and third grids G2 and G3, and a main lens portion (ML) formed by the third and fourth grids G3 and G4.
In general, one of the factors that largely influence the focusing characteristics of a color cathode-ray tube is the aperture of the main lens of an electron gun. As the aperture of the main lens increases, the magnification and aberration can be decreased to obtain a smaller beam spot on a phosphor screen.
In the electron gun for an inline color cathode-ray tube, however, since electrodes corresponding to the three cathodes are integrally placed on the same plane and accommodated in a neck tube whose inner size is limited, the aperture and spacing of electrodes forming the main lens are greatly limited. That is, demands for a larger aperture of the main lens is very difficult to meet.
For this reason, the facing end faces of the third and fourth grids G3 and G4 forming the main lens are constructed by cylindrical peripheral rim electrodes with elliptical sections, and a common lens is formed for three electron beams to obtain a field superposition large-aperture lens.
In this arrangement, however, since the main lens is an elliptical rotation-asymmetrical lens, the horizontal aperture is larger than the vertical aperture, and thus penetration of the electric field is larger in the horizontal direction than in the vertical direction. Accordingly, the lens focuses more strongly in the vertical direction to cause astigmatism in focusing an electron beam.
Jpn. Pat. Appln. KOKOKU Publication No. 2-18540 discloses an arrangement in which a correction electrode plate with noncircular aperture holes through which three electron beams pass and have a hole diameter smaller in the horizontal direction than in the vertical direction is arranged inside the peripheral rim electrode or on its bottom. With this arrangement, horizontal penetration of the electric field can be suppressed to make horizontal and vertical focusing effects uniform and to correct any astigmatism.
On the main lens having this structure, however, penetration of the electric field is smaller for the side beams than for the center beam, and the lens focuses the side beams more strongly. In focusing the electron beams, the side beams suffer more spherical aberration. In each side beam, penetration of an electric field on the outer side (side facing the center beam) is smaller than on the center beam side, and a lens aperture difference is generated. As a result, the beam spot has a shape as shown in FIG. 2. More specifically, while a center beam 30G forms a substantially circular beam spot, side beams 31R and 31B form distorted beam spots with largely spread halos 32R and 32B, causing coma.
To correct the coma, the side beam aperture hole of the main lens must have a diameter as large as possible. However, since the horizontal diameter of the beam aperture hole of the correction electrode is set small in order to correct the above-described astigmatism, the effective diameter of the side beam aperture hole cannot be increased.
Jpn. Pat. Appln. KOKOKU Publication No. 4-44379 discloses an electron gun having an arrangement wherein a correction electrode prepared by removing those portions of its right and left ends which are in contact with a peripheral rim electrode, and forming only an aperture hole through which a center beam passes is arranged to surround the side beams by the end portions of the correction electrode and the peripheral rim electrode. In this electron gun, penetration of an electric field of the side beams on the facing side to the center beam is enhanced, and the aperture of the side beam aperture hole is increased, thereby correcting any spherical aberration and coma.
In this electron gun, however, since those portions of the right and left ends of the correction electrode which are in contact with the peripheral rim electrode are removed, the astigmatism correction effect for the side beams decreases to fail satisfactory astigmatism correction, resulting in a low resolution of the color cathode-ray tube.
In an arrangement disclosed in Jpn. Pat. Appln. KOKAI Publication No. 7-226170, the effective aperture is increased by the following method. In a high-voltage electrode of two cylindrical electrodes constituting a main lens, that surface of the peripheral wall forming an opening, which faces a low-voltage electrode, has a shape defined by upper and lower straight lines parallel to the inline direction and having their respective terminating points connected to circular arcs having the tube axis as a center. In the low-voltage electrode, that surface of the peripheral wall forming an opening, which faces the high-voltage electrode, has a shape defined by upper and lower straight lines parallel to the inline direction and having their respective terminating points connected to elliptical arcs curved outward.
In this case, although the lens aperture for the side beams can be increased, astigmatism occurs in the horizontal, vertical, and diagonal directions of the side beams. For this reason, if astigmatism is corrected by the correction electrode, as described above, the lens aperture for the side beams decreases. In addition, coma easily occurs for the side beams. Accordingly, desired lens performance cannot be obtained owing to two contradiction demands for increase a lens aperture and correction of astigmatism.